Studies about a method for preparing methacrylic acid with high selectivity have been carried out concerning catalysts used in the methacrylic acid preparation by gas phase oxidation of methacrolein and among patent literatures. The main categories of the inventions are about the heteropoly acid catalysts since it is known to show good activity.
The heteropoly acid, however, has many disadvantageous basic properties for solid catalysts having insufficient catalytic capacity. Consequently it was difficult to apply commercially. The main disadvantages of the heteropoly acid catalysts are difficulties in getting consistent results, weak mechanical strength, and short life cycle of the catalysts. Other disadvantages to overcome are that the conversion rate and selectivity sharply decrease when unsaturated hydrocarbon and aromatic compounds are contaminated in the reactants. Mostly, the methacrolein is obtained by a catalyzed oxidation of isobutylene, wherein unreacted isobutylene and byproducts such as toluene and xylene etc., affect the capacity of the catalysts for the preparation of methacrylic acid thereby reducing the conversion rate and selectivity.
Many attempts to overcome said disadvantages have been disclosed in patent specifications. For example, there was an attempt to increase the selectivity of the catalysts by adding arsenic in the catalyst composition, but the problem was that the life cycle of the catalyst was short because of sublimation properties in the arsenic. There was another attempt to ensure the consistency of the catalysts and to increase the selectivity for methacrylic acid (European Patent Publication 0454376, Japanese Patent Publication So 57-171443, U.S. Pat. No. 4,621,155) and these purposes have been satisfied. The problem, however, was that the catalysts have low mechanical strength and the selectivity for methacrylic acid is low when the methacrolein obtained from oxidation of isobutylene, not the pure methacrolein, is used.
Furthermore, to increase the mechanical strength of the catalysts, for example, the mechanical strength can be increased by using sulfuric acid salts of metal. The problem was the capacity of the catalysts is too low (Japanese Patent Publication So 55-79340). The strength can also be increased by adding ceramic whiskers such as silicon carbide or silicon nitride etc., but there are many difficulties in applying it in reality because the ceramic whiskers are too expensive.
It is very important to maintain the selectivity for methacrylic acid particularly because the preparation method for methacrylic acid is commonly composed of a recycling process of the unreacted methacrolein due to instability of methacrolein and low heat stability of heteropoly acid catalysts.